In many wireless communications systems, the availability of channel information (e.g., channel quality information, channel state information (CSI), power control information, etc.) is crucial for obtaining good performance. The channel information can be determined, for instance, by estimating a channel state from a transmitted reference signal (RS). The estimated channel state can then be used to report Ca In Long Term Evolution (LTE), CSI typically includes channel quality indicator (Cal), rank indicator (RI), and pre-coding matrix indicator (PMI) values.
The 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system supports CSI reporting schemes that rely on reference signals being transmitted periodically. For instance, cell-specific reference signals (CRS) are sent every subframe while user-specific demodulation reference symbols (DM-RS) and channel state information reference signals (CSI-RS) can be sent with a longer periodicity. User equipment (UEs) using transmission mode 10 (TM10) rely solely on CSI-RS resources while other UEs typically use the CRS resources at least for interference measurements.
TM10 UEs may be configured to report CSI for multiple CSI processes, with each UE possibly having different CSI-measurement resources. A CSI-measurement resource (CSI-MR) consists of a CSI-RS resource and a CSI interference measurement (CSI-IM) resource. Both the CSI-RS and the CSI-IM resources are divided into sets of resources, where each set is identified by a CSI-RS configuration index. Each CSI-RS/CSI-IM configuration index indicates resources in physical resource blocks (PRBs) in the frequency band. A subframe configuration specifies a subframe periodicity and a subframe offset that defines at which time instances the respective measurement resources are available for the UE.
Time-based filtering or averaging of interference is sometimes advantageous when the interference variations are unknown to a network node (e.g., base station) serving a UE, while it is disadvantageous when the variations may be predicted by the serving network node. To improve performance of coordination features, the UE may be configured to not perform time-based filtering or averaging of the interference estimated on the CSI-IM resource. Accordingly, the reported CSI will reflect the instantaneous quality of the channel at the time of the measurement.
In 5th generation mobile network (5G) systems and LTE systems there is support for multiple numerologies. A numerology is a supported configuration of an orthogonal frequency division multiplexing (OFDM) radio interface such as a certain sub-carrier spacing. Further, a numerology may be related to sub-carrier spacing with a decreasing numerology corresponding to an increasing sub-carrier spacing and an increasing numerology corresponding to a decreasing sub-carrier spacing. However, while a 5G system is expected to operate multiple numerologies simultaneously, an LTE system may operate only one numerology at a time. In OFDM, there is a fixed relationship between the sub-carrier spacing (SC) and the OFDM symbol time duration (T) such that the product of the sub-carrier spacing and the OFDM symbol duration is constant (i.e., SC×T=Constant).
In addition to this relationship, the time duration of the cyclic prefix (CP) should also be considered since the total time between two OFDM symbols constitutes both the OFDM symbol duration and the duration of the CP. If the duration of the CP is kept constant, then the overhead from the CP increases when the OFDM symbol duration is decreased. Alternatively, if the duration of the CP and the OFDM symbol duration are proportionately adjusted, then the duration of the CP decreases when the OFDM symbol duration is decreased. For instance, when the sub-carrier spacing is doubled, the durations of the CP and the OFDM symbol are halved. This implies that when increasing numerology, the duration of the CP becomes shorter, which decreases the margin for delay spread and propagation delay. Alternatively, when decreasing numerology, the duration of the CP becomes longer, which increases the margin for delay spread and propagation delay.
In future wireless systems such as 5G, the network will support a larger set of numerologies (such as different options for the sub-carrier spacing). In this case, the number of resources for measuring CSI can also multiply. For instance, for two simultaneous numerologies, the network will need to transmit CSI-RS and CSI-IM for each numerology. Also, the UEs will need to perform twice as many measurements, resulting in higher power consumption and lower performance by the UEs depending on how efficiently each UE can switch between numerologies.
Accordingly, there is a need for techniques to reduce the impact from simultaneously supporting multiple numerologies.
The Background section of this document is provided to place embodiments of the present disclosure in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.